Generally, in plasma arc welding, a strong plasma gas flow is used to melt a base metal. Therefore, for butt welding in plasma arc welding, keyhole welding is generally performed to prevent weld defects such as humping beads. Therefore, the depth of penetration is larger than that in TIG welding, and the welding speed is higher. Further, in plasma arc welding, the bead width is small, and this is advantageous in that strain in the base metal is small. The keyhole welding is a method in which welding is performed while a keyhole (small hole) passing through the sheets in their thickness direction is formed by the plasma gas flow. For example, Patent Literature 1 discloses martensitic stainless steel containing 7 to 14 mass % of Cr and controlled amounts of Al, P, S, and O in order to perform melt welding such as plasma arc welding.
As for keyhole welding (hereinafter referred to also as plasma keyhole welding) for stainless steel, keyhole welding has been conventionally used for austenitic stainless steel typified by SUS304. However, austenitic stainless steel has a drawback in that stress corrosion cracking is likely to occur, so that ferritic stainless steel that is less likely to undergo stress corrosion cracking is used in not a few applications. Therefore, there is a need to perform plasma keyhole welding also on ferritic stainless steel.
Patent Literature 2 discloses a welding method in which ferritic stainless steel with a thickness of 3 mm or less is subjected to non-keyhole welding using a plasma arc welding torch. Therefore, this method is not for keyhole welding.